Cancer is a leading cause of premature deaths in the developed world. Immunotherapy of cancer aims to mount an effective immune response against tumour cells. This may be achieved by, for example, breaking tolerance against tumour antigen, augmenting anti-tumor immune responses, and stimulating local cytokine responses at the tumor site. The key effector cell of a long lasting anti-tumor immune response is the activated tumor specific effector T cell. Potent expansion of activated effector T cells can redirect the immune response towards the tumor. In this context, regulatory T cells (Treg) play a role in inhibiting the anti-tumor immunity. Depleting, inhibiting, reverting or inactivating Tregs may therefore provide anti-tumor effects and revert the immune suppression in the tumor microenvironment. Further, incomplete activation of effector T cells by, for example, dendritic cells can cause T cell anergy, which results in an inefficient anti-tumor response, whereas adequate induction by dendritic cells can generate a potent expansion of activated effector T cells, redirecting the immune response towards the tumor. In addition, Natural killer (NK) cells play an important role in tumor immunology by attacking tumor cells with down-regulated human leukocyte antigen (HLA) expression and by inducing antibody dependent cellular cytotoxicity (ADCC). Stimulation of NK cells may thus also reduce tumor growth.
OX40 (otherwise known as CD134 or TNFRSF4) is a member of the TNFR family that is expressed mainly on activated T cells (mostly CD4+ effector T cells, but also CD8+ effector T-cells and regulatory T cells (Tregs)). In mice the expression is constitutive on Tregs, but not in humans. OX40 expression typically occurs within 24 hours of activation (T cell receptor engagement) and peaks after 48-72 hours. OX40 stimulation is important for the survival and proliferation of activated T cells. The only known ligand for OX40 is OX40L, which is mainly expressed on antigen presenting cells, such as dendritic cells and B cells, typically following their activation. The net result of OX40-mediated T cell activation is the induction of a TH1 effector T cell activation profile and a reduction in the activity and/or numbers of Treg cells e.g. via ADCC or ADCP. Overall these effects may contribute to anti-tumor immunity. OX40 is overexpressed on regulatory T cells in many solid tumors, such as melanoma, lung cancer and renal cancer.
OX40 agonist treatment of tumor models in mice has been shown to result in anti-tumor effects and cure of several different cancer forms, including melanoma, glioma, sarcoma, prostate, colon and renal cancers. The data is consistent with a tumor specific T-cell response, involving both CD4+ and CD8+ T cells, similar to the effect seen with CD40 agonist treatments. Addition of IL-12 and other cytokines, and combination with other immunomodulators and chemo/radiotherapy, has been shown to improve the therapeutic effect of OX40 agonist treatment. Evidence from pre-clinical models suggests that the effect of anti-OX40 antibodies is dependent upon activating FcγR. A clinical phase I study testing the mouse anti-human OX40 Clone 9B12 in late stage patients that had failed all other therapy has been conducted at the Providence Cancer Centre. The antibody was well-tolerated. Tumor shrinkage and an increase in CD4+ and CD8+ T cell proliferation were observed. The low toxicity may be caused by low half-life and anti-drug antibodies (the antibody was a mouse antibody), but also by the relatively low expression levels of OX40 on non-activated T cells. The anti-tumor effect with this antibody was modest.
CD137 (4-1BB, TNFRSF9) is also a member of the TNFR family. Activation of CD137 is dependent on receptor oligomerization. CD137 is expressed on activated CD4+ and CD8+ T cells, Treg, DC, monocytes, mast cells and eosinophils. CD137 activation plays an important role in CD8+ T cell activation and survival. It sustains and augments, rather than initiates effector functions and preferentially supports TH1 cytokine production. In CD4+ T cells, CD137 stimulation initially results in activation and later in activation-induced cell death, which may explain why CD137 agonistic antibodies have shown therapeutic effect in tumor immunity as well as in autoimmunity. CD137 also suppresses Treg function. CD137 is upregulated on NK cells activated by cytokines or CD16. Activation of CD137 on NK cells has been shown to increase ADCC activity of NK cells in both murine and human cells. Further, CD137 is expressed on antigen presenting cells, such as dendritic cells and macrophages, and stimulation of CD137 on these cell types may induce immune activation that can result in tumor directed immunity. CD137 agonistic antibody has also been shown to activate endothelial cells in the tumor environment, leading to upregulation of ICAM-1 and VCAM-1 and improved T cell recruitment. Several studies have demonstrated induction of tumor immunity by treatment with agonistic CD137 antibodies.
Two CD137 antibodies are in clinical development. Urelumab (BMS-66513) is a fully human IgG4 antibody developed by Bristol-Myers Squibb. Several phase I and II studies in various indications are currently ongoing. A Phase II study with Urelumab as a second line therapy in metastatic melanoma was terminated in 2009 due to fatal hepatoxicity. The other CD137 mAb in clinical development is PF-05082566, a fully human IgG2 antibody developed by Pfizer. It is currently in phase I development in lymphoma and various solid cancers and preliminary data suggest that it is well tolerated but with only modest anti-tumor effects.
Existing antibodies targeting CD137 or OX40 are in general dependent on cross linking via e.g. Fcgamma Receptors on other cells to induce strong signaling into cells expressing the respective receptor. Thus, they do not signal efficiently when no such cross linking is provided. In addition, prolonged and continuous activation through TNF receptor family members may lead to immune exhaustion.
The T cell receptor CTLA-4, serves as a negative regulator of T cell activation, and is upregulated on the T-cell surface following initial activation. The ligands of the CTLA-4 receptor, which are expressed by antigen presenting cells are the B7 proteins. The corresponding ligand receptor pair that is responsible for the upregulation of T cell activation is CD28-B7. Signalling via CD28 constitutes a costimulatory pathway, and follows upon the activation of T cells, through the T cell receptor recognizing antigenic peptide presented by the MHC complex. By blocking the CTLA-4 interaction to the B7-1 and, or B7-2 ligands, one of the normal check points of the immune response may be removed. The net result is enhanced activity of effector T cells which may contribute to anti-tumour immunity. As with OX40, this may be due to direct activation of the effector T cells but may also be due to a reduction in the activity and/or numbers of Treg cells, e.g. via ADCC or ADCP. Clinical studies have demonstrated that CTLA-4 blockade generates anti-tumor effects, but administration of anti-CTLA-4 antibodies has been associated with toxic side-effects. CTLA-4 is overexpressed on regulatory T cells in many solid tumors, such as melanoma lung cancer and renal cancer.
There is a need for an alternative to the existing monospecific drugs that target only one T cell target, such as OX40 or CD137 or CTLA-4.